[Rev]

My first thought is a head gasket leak. If coolant is disappearing without any visible puddle under the car and no steam emminating from the engine, well the coolant has to be going somewhere. Look for white smoke out the exhaust. That's really steam from the combustion chamber, and always shows either a head gasket leak or a crack in the head or block.

I am somewhat surprised that the pressure chek on the heads showed no leak down from the coolant passages since that coolant has to be going somewhere. It is making it's way into the oil. It must be being forced past the rings or from a cracked cylinder into the bottom end.

My bet is the head gasket.

Rev